fix: do not close request right after sending it, assume it may have to be retried

with the retries plugin, the request payload will be rewinded, and that may not be possible if already closed. this was never detected so far because no request body transcoder internally closes, but the faraday multipart adapter does

the request is therefore closed alongside the response (when the latter is closed)

Fixes https://github.com/HoneyryderChuck/httpx/issues/75\#issuecomment-2731219586